Reinforcement elements of this type are principally, but not exclusively, employed in the aeronautic and space fields in which they have many applications, in particular for producing parts which must resist thermo-mechanical stresses, such as thermal protections of bodies re-entering the atmosphere, explosive-driven rocket nozzles, aircraft brakes, or parts which must withstand high mechanical stresses, such as the hubs of helicopter rotors, landing undercarriages, roots of wings, leading edges, etc.
Many processes and apparatus have been imagined and developed for producing such reinforcement elements, but the automatized manufacture of parts of complex shape encounters great difficulties which result in very complicated and consequently costly machines without the parts obtained always possessing the required qualities of homogeneity and resistance.
Moreover, the remarkable properties of these composite elements lead to the use thereof for producing parts having complicated, evolutive shapes that present machines are incapable of manufacturing.
It is known to produce hollow, composite reinforcement elements of revolution woven in two dimensions horizontally around rigid, perpendicular rods mounted in concentric ring arrangements on a rotatable support, which are subsequently replaced by threads, as described for example in U.S. Pat. Nos. 4,183,232 and 4,346,741 filed in the name of the applicant.
According to another method, a hollow support mandrel is used, parallel layers of threads are laid in two crossed directions on the surface of the mandrel and stitching lines are formed in a direction perpendicular to these layers, as described in particular in No.FR-A-2,355,936.
According to No. FR-A-2,315,562, the hollow support mandrel is of metal, capable of being taken apart, and formed by spaced-apart sectors having apertures in which are driven points about which are stretched out threads forming the different superimposed crossing layers which are thereafter sewn by rows of stitches formed in the gaps between the sectors of the mandrel.
All these processes disclosed in these documents require a hollow mandrel since the connection of the superimposed layers by stitching necessarily results in the introduction in the mandrel of a device for knotting the thread introduced from the exterior.
Moreover, the stitches are effected with needles with flaps or closed eyes which are delicate to use for fragile fibers requiring sometimes a double lapping of the thread.
Another process disclosed in No.FR-A-2,408,676 on the other hand employs a solid mandrel of foam material on which are mounted sections of rigid threads, termed "picots" around which the layers of threads are laid in two different directions and which constitute the threads of the third direction.
This process has various drawbacks. First of all, the "picots" must be previously subjected to a pre-rigidifying treatment, which increases their diameter, to permit the implantation thereof.
Secondly, the "picots" which must become an integral part of the part to be produced must consequently be provided in a considerable number, on the order of several tens of thousands, implanted very close together, which represents an extremely long operation requiring high precision.
Furthermore, in the case of a part having a complex shape whose surface forms corners or curves, the implantation of the neighboring "picots" which are excessively close together, is very difficult to achieve without interference therebetween, and the very narrow passageways defined therebetween do not permit an easy laying of threads in even layers, which laying is even found to be impossible in the regions where the threads change orientation.
Lastly, the "picots" excessively close together behave imperfectly, in particular in the curved parts, which results in defects in the homegeneity in the finished part.